Why do crowns fail?

The preparation, impression and provision of crowns is an essential procedure in contemporary dentistry. With the advent of new ceramics, bonding materials and preparation techniques the situations in which crown and bridge work can be utilised is broader than ever. However, with these developments comes increased procedural complexity which can lead to treatment failure. Crowns can fail for any number of reasons some of which are explored below:

Structural failure: Failure in the crown or tooth substrate

• Poor preparation technique can result in failure of the crown.
• Insufficient tooth reduction will result in thin sections or restorative material that can easily fracture. Use depth cutters or calibrated burs to the reduction that you want. If the temporary crown fractures quickly this may be a sign that you have insufficient reduction
• Sharp angles in the crown preparation will make impressions and stone pouring difficult as the material has difficulty flowing around sharp corners. This may lead to an ill fitting crown. Sharp internal line angles in the crown will lead to stress concentrations and catastrophic fracture after repetitive chewing cycles. Assess your prep before the impression stage and use a polishing disc to round off any sharp angles.
• Wrong bonding protocol: designing cement retained vs bonded restorations usually relies on the availability of moisture control and a 360 degree ring of enamel. However, material choice comes into consideration. Most restorations can be bonded in but for PFMs and zirconias there usually is minimal benefit provided there is sufficient retention in the prep. However, Lithium disilicate restorations are of insufficient strength unless they are bonded to tooth structure. Early failure of a LiSi crown can occur if you attempt to cement it in place.
• Poor lab technique: The laboratory must adhere to the material guidelines from the manufacturer. Incorrect firing of porcelain or poor glazing can result in a weak restorative surface that is prone to fracture. Go to your laboratory and observe how they work and the steps required in the production process. You will learn that visualising the end result is much more difficult in the lab as the patient is not there for reference. What will look good in the lab can just as easily look horrible once placed in the mouth. Hopefully you will learn what instructions and records the laboratory requires to produce good quality work.
• Failure at core-tooth interface: The crown can dislodge if there is a failure at the core-tooth interface. Ensure you have good moisture control i.e rubber dam and your bonding protocols are up to standard. Using a separate self etching prime and bond system is the gold standard. In cases there there is insufficient tooth structure to retain a core, consider placing a post in the root canal system. you must ensure there is sufficient ferrule to support the crown or all the occlusal lateral forces will be placed on the post leading to early fracture of the root.
• Insufficient ferrule: 2mm of tooth structure circumferentially is suggested to reduce the risk of crown dislodgement and root fracture. If there is insufficient ferrule, consider crown lengthening, orthodontic or surgical extrusion or vertiprep techniques.
• Overpreparation of the root canal system: Conservation of pericervical dentine should be one of the primary aims of root canal treatment. Over preparation of the root canal system with large access cavities and overtapered files increases the risk of a catastrophic vertical root fracture.
• Insufficient support of porcelain: Porcelain requires rigid support for strength as it is a brittle material. This is why it is best enamel supported porcelain veneers are so strong. Thick sections of weaker feldspathic porcelain due to poor coping design can lead to fractures especially on the marginal distal ridges of molars. Extend the coping almost to the height of contour in proximal surfaces to ensure that the porcelain on the marginal ridges is well supported.
• Rough surface of porcelain: This leads to crack propogation especially in zirconia. Glazing should be done by the technician and if any occlusal adjustments are made they should be polished well under water spray.

Interface failure: Failure of bonding or cement

•  Poor bonding technique is the main cause of interface failures
• Plaque contamination of the bonding surface: Pellicle and biofilm coatings on teeth are high in protein and hinder the penetration of acids and bonding agents. Plaque disclosing should be performed to identify areas of plaque buildup and sandblasting or pumicing should be performed to clean the surface. Acid etch will eventually penetrate the plaque substrate but this effect is unpredictable and there will be sites of poor bond strength
• Rubber dam isolation is ideal to stop ingress of saliva and gingival crevicular fluid. Moisture contamination of the tooth or crown surface will hinder the penetration of hydrophobic bonding agents.
• Bonding lithium disilicate requires etching with hydrofluoric acid, if this is not performed correctly, there can be no resin penetration into the ceramic matrix and no bonding will occur. 
• Currently, bonding to zirconia is possible but unreliable. It involves chemical bonding of resin to phosphate groups in the zirconia matrix. Consequently, contamination of the intaglio surface with phosphoric acid will fill all the available bonding groups and remove the ability to bond to the zirconia forever.
• Follow the manufacturer's instructions for all bonding steps otherwise you risk a substandard bond. Ensure complete seating of the crown as incomplete seating will result in a thick luting layer which is the weakest part of the system.
• Demineralised enamel at margins or incomplete removal of caries will result in poor bond strength at this site. This is because an unpredictable etching pattern will be produced at this side. Either take the crown margin below any demineralised enamel or put the patient through a remineralisation protocol before preparation
• Resin bonding to dentine is unpredictable long term due to higher organic material and moisture content. Therefore, bond strength to dentine tends to decrease over time.
• All cements will lose about 50% of their strength after 10 years due to breakdown of the components. After this point, resistive features of the tooth preparation become more important in reducing the strain on the cement.
• Relying on bond strength is not recommended without complete enamel margins. Attempting to bond to partial enamel margins will lead to an unpredictable long term bond and instead, retentive features should be included such as sufficient height of preparation and interproximal grooves or boxes. Failure to include these in cemented restorations will results in dislodgement of the crown.

Biological failure: Failure due to factors in the health of the tooth or mouth

• Sensitivity after bonding: This may happen for a few reasons
• If there is sensitivity to bite after insert but not in the temporar crown stage
• Poor bonding technique allowing micromovements and leakage under the crown. Remove the crown and replace with a temporary to see if symptoms resolve. This can be due to over etching of dentine, poor penetration of bonding agents, insufficient cleaning of tooth surface, insufficient curing of cement
• Occlusion factors: A high spot in the occlusion due to poor design or overeruption of teeth from a poorly shaped temporary crown or a lateral interference due to poor design of the crown can cause periodontal ligament inflammation and pulpitis. Check the occlusion and adjust as needed
• If there is pain after tooth preparation and in the temporary crown stage: It is likely that the tooth has developed a pulpitis. This is unlikely if you have not exposed the pulp and used sufficient water spray however the risk of this is higher in younger patients where the pulp space is much larger. Ensure you have good diagnosis of the pulp and periapical areas before crown preparation is commenced. Pulp sensibility tests and a periapical xray are recommended. Preoperative consent is important and root canal treatment is suggested. In borderline cases, consider placing a long term temporary crown e.g Acryllic crown with permanent cement or using a material such as gold that can be accessed through without weakening the whole structure. In young patients, consider a more conservative restoration until the pulp has time to receed.
• Localised periodonitis: Poor cement cleanup or overhanging margins will result in localised periodntal inflammation with risk of long term attachment loss. Ensure your preparations and impressions are ideal to ensure a well fitting margin. Clean the cement well and consider taking a bitewing radiograph after cementation to check if there is any residual cement inteproximally. 
• Dental caries: Mainly arising from poor oral hygiene and dietary factors. However poor crown design including overcontoured smooth surfaces can reduce the cleansing action of food and the cleansability of the crown. Tight contacts make oral hygiene difficult and open contacts can result in food packing and stagnation. Poor marginal integrity can occur due to poorly fitting crowns or failure to remove demineralised enamel at the margin.

Aesthetic failure
auesthetic failure: reduction of tooth, shade shape communication with technician supragingival margins
metal showthough opaque pfm copings,
dark tooth showthrough of porcelain crowns
root showthrough high smile line
poorsoft tissue management of zeniths, smile design
parafunction and design of crown